(1) Field of the Invention
This invention relates generally to a method and apparatus for processing optical waveguides and more particularly to a method and apparatus for irradiating patterns in such optical waveguides.
(2) Description of the Prior Art
Optical waveguides, including planar waveguides, fiber and fiber-like substrates such as fiber optic cable, are known. These waveguides may comprise a central, light transmissive, cylindrical or semi-cylindrical glass core surrounded by a light reflecting or retracting transmissive glass cladding. Such waveguides may include additional rings, semi-rings or layers of fusible glass or other refractive, reflective or protective materials.
A planar waveguide may incorporate all of the above attributes and may comprise additional attributes. They are normally formed in flat sheet(s) of glass or other known optical radiation transmissive materials. These waveguides may also be formed on non-flat sheet(s) of materials or on the surface of non-optically transmissive materials. The form of these waveguides are known to be substantially different from that commonly found in optical fibers. Various methods for forming these planar waveguides are known including chemical vapor deposition, sputtering, electron beam or ion beam implantation. These methods or combinations of methods allow much more complex patterns to be formed in a much more compact manner that are possible with optical fiber type waveguides.
It is also known to change the useful properties or characteristics of selected areas of such waveguides. For example, the following United States Letters Patent disclose waveguides with altered light transmission characteristics and methods for making such alterations:
______________________________________ 3,916,182 (1975) Dabby et al. 4,182,664 (1980) Maklad et al. 4,400,056 (1983) Cielo 4,403,031 (1983) Borrelli et al. 4,636,031 (1987) Schmadel, Jr. et al. 4,776,661 (1988) Handa 4,725,110 (1988) Glenn et al. 4,793,680 (1988) Byron 5,042,897 (1991) Meltz et al. 5,061,032 (1991) Meltz et al. 5,066,133 (1991) Brienza 5,104,209 (1992) Hill et al. ______________________________________
The Dabby et al. patent discloses an optical waveguide comprising either a substrate coated with a layer of optic material or a clad optic fiber. A periodic variation in the index of refraction of either the substrate and/or the optical layer, the core and/or cladding of the optic fiber is introduced so that unwanted frequency components present in the optical signal passing through the waveguide are eliminated. The waveguide may be employed as a band-pass or a band-stop filter or for phase-matching purposes. Various means are disclosed for altering the periodicity of the index of refraction to thereby tune the device.
In the Maklad et al. patent optical fibers of silica and plastic composition are rendered relatively stable to nuclear radiation induced optical losses by pre-irradiating with a high initial radiation dose. Subsequent exposure of the radiation hardened fibers produces a substantially lower radiation induced optical loss and faster fiber transmission recovery rates.
The Cielo patent discloses a tunable optical fiber reflector together with a method of making such a reflector. A length of optical fiber has a core of a first light transmitting material, and a cladding of a second light transmitting material covering the core. The cladding is etched away to a predetermined thickness over a portion of the fiber. A layer of photoresist material is applied either to the etched portion of the fiber or to a thin metal blade, i.e., a mask, and then exposed to beams of light that optically interfere and generate a standing wave pattern in the material. The photoresist material is then developed to fix the wave pattern in the material. An optical discontinuity is formed in one of the core and cladding by that fixed wave pattern when the photoresist is on the etched section or when the developed mask is used to expose the core and cladding. This discontinuity represents a quasi-periodical fluctuation in the refractive index and causes evanescent waves in the cladding to be reflected. Such a discontinuity forms a distributed-feedback reflector.
In accordance with the Borrelli et al. patent optical patterns formed by localized optical density or refractive index variations in glass are produced by impregnating a porous glass support with a photolyzable organometallic compound and selectively exposing the glass to a photolyzing light source to cause the photolytic decomposition of the organometallic compound in exposed portions of the glass. The patterns are fixed, if desired, by removing unreacted organometallic compound from the pores.
The Schmadel, Jr. et al. patent discloses a tuned optical fiber grating and a tuning process. The gratings on the optical fiber are tuned so that the reflectance of the grating can occur at a specific wavelength. The process involves encasing that portion of the fiber containing the grating while shining light of the wavelength of desired reflectance through the fiber and stretching the grating until reflectance occurs. Thereafter, the tuned grating is sealed within a tube formed around the tuned grating.
An integrated optical device shown in the Handa patent performs optical data processing in an integrated arrangement using an optical waveguide. The waveguide comprises a substrate, a slab optical waveguide provided on the substrate, a channel optical waveguide provided at a portion of the slab optical waveguide, and a grating coupler provided with a grating structure at a portion of the channel optical waveguide to optically couple the slab optical waveguide and the channel optical waveguide.
In the Glenn et al. patent a dielectric periodic index of refraction phase grating is established upon the core of an optical waveguide by an intense angled application of several transverse beams of ultraviolet light. This enables the establishment of a distributed, spatially resolving optical fiber strain gauge.
The Byron patent discloses the use of a pulsed high-power laser beam incident on the surfaces of a wide variety of materials to produce ripples on such surfaces. These ripples result from an interference between scattered waves and an incident beam producing intensity fringes, and hence localized heating. The dimensions of the ripples are dependent on the wavelength of the incident light. Hence a grating whose length is a few hundreds of micrometers is produced. If the cladding is removed this effect of ripple generation is enhanced.
Optical waveguides disclosed in the Meltz et al. patents incorporate Bragg diffraction gratings. In both patents the grating element is formed in the core or the waveguide by exposing the core or waveguide to an interference pattern of two ultraviolet radiation beams that are symmetrical with respect to a plane extending at the oblique angle relative to the core or waveguide axis.
Apparatus for forming an extended length of Bragg gratings in an optical waveguide, as disclosed in the Brienza patent, includes a source that directs a coherent light beam of a frequency in the ultraviolet range in a primary path transversely toward the waveguide. A section of a diffraction grating extends through the primary path at a spacing from the waveguide, and the diffraction grating has a dimension normal to the primary path. Relative movement is effectuated between the waveguide and diffraction grating in unison and the primary path. Consequently, the light beam diffracts at the diffraction grating into two mutually frequency-shifted partial light beams propagating in diverging secondary paths. The partial light beams are caused to travel toward a shared location of the waveguide where they form an interference pattern that moves longitudinally of the waveguide but respective high intensity fringes of which extend through the waveguide at respective positions that are stationary relative to the waveguide to effect refractive index changes at such positions along an extended length of the waveguide.
The Hill et al. patent discloses a method of creating a grating in an optical fiber. This method comprises disposing a slit mask containing one or more slits over a side of an optical fiber and illuminating the fiber through the slit mask by substantially monochromatic ultraviolet light for a short interval, whereby an index grating line is created and stored in the core of the fiber.
The Dabby et al., Maklad et al., Cielo, Borrelli et al., Schmadel et al. and Handa patents alter characteristics by photolithographic, mechanical, chemical and other related processes. The Glenn, Byron, Meltz et al., Brienza and Hill et al. patents disclose generally the formation of a Bragg grating by irradiating an optical waveguide with light in particular bandwidths. Light in the green and ultraviolet spectra are particularly used normally with germania- or alumni-doped optical waveguides.
Bragg gratings established by such methods are of limited use as such gratings only refract light frequencies in limited frequency bands. These bands generally constitute only a portion of spectra used in most applications. Further, these methods produce gratings with germania doped fibers and normally along the entire length of the core fiber only.
Moreover, the processes disclosed by these references require both doped fibers reactive to ultraviolet light and lasers for generating ultraviolet light. Non-predictable variations in the level of photobleaching can occur particularly at boundaries between areas of normal light transmission. When this occurs, the refraction near the selected frequency can vary across a boundary area. Such variations can require tuning as described in the Schmadel et al. patent and can limit the usefulness of the fiber. Moreover, controls to limit such variations increase processing complexity and cost.
The references fail to teach a method and apparatus for forming patterns and particularly Bragg gratings in optical waveguides to refract or reflect light of a selected frequency at one or more predetermined cable locations. The references also fail to disclose a relatively simple procedure and apparatus for forming Bragg gratings that can be formed for refracting desired frequencies of light. Additionally, these references fail to provide a method and apparatus for forming patterns in a plurality of segments of a single or plurality of waveguides in a relatively simple and efficient manner. Finally, the references fail to teach or suggest apparatus and a method for forming patterns in optical waveguides formed of a wide variety of materials usable for producing optical waveguides.